The present invention relates to a process for treating a chromate waste liquid, which is produced in a chromate treatment line, and particularly to a process for removing a dissolved chromium component from the chromate waste liquid.
It is known to form an anticorrosive Zn or Sn plating layer on a surface of various iron-based parts such as metal bodies of spark plug and glow plug. Furthermore, a chromate film may be formed on such plating layer by a chromate treatment, when a higher anti-corrosiveness is required. For example, a zinc plating layer is able to provide an iron surface with superior anti-corrosiveness. It is, however, well known that such zinc plating layer tends corrode by a so-called sacrifice corrosion. The zinc oxide resulting from this corrosion has a white color, thereby making the external appearance inferior. Thus, it is often to cover the zinc plating layer with a chromate film in order to prevent corrosion of the plating layer.
Yellow chromate has widely been used for the chromate treatment, since it is good in anti-corrosiveness. A part of chromium component of yellow chromate, however, is in the form of hexavalent chromium. Therefore, it is a recent trend to avoid the use of such yellow chromate from the viewpoint of environmental protection. Since a yellow chromate treatment bath contains hexavalent chromium of a relatively high concentration, it is necessary to conduct a reducing treatment for reducing the hexavalent chromium to trivalent chromium. This increases the chromate treatment cost.
As an alternative chromate treatment, a chromate treatment with trivalent chromium, in place of hexavalent chromium, has been attracting much attention in recent years. German Patent Laid-open Publication DE 19638176 A1 discloses a process for forming a condensed, thick, trivalent chromium layer, using a trivalent chromium salt and a complexing agent containing an organic acid as its major component.
It is necessary to treat a chromate waste liquid such as (1) a waste liquid generated by washing a work after its chromate treatment and (2) a chromate treatment liquid that has reached its lifetime. In fact, it is necessary to remove a dissolved chromium component from such chromate waste liquid and then to discard the liquid as an effluent. It is known to conduct this removal by adding Ca(OH)2 to a chromate waste liquid, thereby turning a dissolved chromium component into Cr(OH)3 in the form of precipitate. However, in case that a chromate treatment liquid contains a large amount of an organic acid component (as a complexing agent or the like), as disclosed in the above-mentioned German Patent Publication, the chromate waste liquid also contains a large amount of the organic acid component. This organic acid component may be strongly bonded with chromium. Thus, it may be difficult to separate and remove such chromium component from the waste liquid.